1. Field of the Invention
The field of the present invention is that of optical transmission systems and more precisely that of the amplification of signals in optical transmission systems.
2. Description of the Prior Art
A fiber optic transmission system typically comprises a sender that injects signals to be transmitted into an optical fiber and a receiver that receives the signals after propagation in the fiber. Propagation in the fiber leads to attenuation of the signal. Repeaters may be provided along the fiber to amplify or format the signals.
Using the Raman effect to amplify the transmitted signals has been suggested. Pumping light is injected into the fiber. The power of the pumping light is transferred to the signal by the Raman effect along the fiber. The wavelength of the pumping light is chosen so that the maximum Raman gain occurs near the wavelength of the signal to be amplified. The pumping efficiency is a function of the power of the pumping light and the power of the signal to be pumped. This solution is described in “Nonlinear Fiber Optics”, Govind P. Agrawal, Ed. 2-Chap. 8, for example.
U.S. Pat. No. 6,181,464 (Tycom), entitled “Low noise Raman amplifier employing bidirectional pumping and an optical transmission system incorporating the same”, H. D. Kidorf, proposes simultaneously injecting into the transmission system first pumping light (first pump) P1 at a first pump wavelength λ1 and second pumping light (second pump P2) at a second pump wavelength λ2. The wavelength of the first pump is chosen to pump the signal by the Raman effect and the wavelength of the second pump is chosen to pump the first pump by the Raman effect. FIG. 1 is a graph of the power of the signal S and the pumps P1 and P2 in a system of the above kind in the case of contrapropagating pumps. The power is plotted on the ordinate axis and the distance along the optical fiber is plotted on the abscissa axis. The signal S is injected at one end of the fiber—the left-hand end in the figure—and the pumps are injected at the other end of the fiber—the right-hand end in the figure. The curve marked S in the FIG. 1 graph is the curve of the power of the signal; as the figure shows, the power of the signal is at a maximum on injection and decreases over a first portion of the fiber, as far as a distance d1, because of attenuation in the fiber. The power of the signal is at a minimum at the distance d1. Between the distances d1 and d2, the power of the signal increases because of the pumping of the signal by the pump P1. Beyond the distance d2, the power of the signal decreases again, the power of the pump P1 being insufficient for the pumping to compensate the attenuation of the signal.
At the other end of the fiber, the first pump is injected at a lower power than the second pump. In the pump propagation direction—from right to left in the FIG. 1 diagrams—the power of the second pump decreases because of attenuation and because of the transfer of power to the first pump by the Raman effect. In the pump propagation direction, the power of the first pump initially increases, the effects of attenuation being compensated by the pumping of the first pump by the second pump. The power of the first pump thereafter decreases, because of the transfer of power from the first pump to the signal by the Raman effect and because of attenuation by the fiber.
The above system has the following drawbacks. The pumping power injected into the fiber is limited by Rayleigh backscattering and limits the injection of power from the first pump. For a given fiber, it is not possible simply to choose the region in the fiber in which the signal will be amplified or to modify this region after the system has been designed.
There is therefore a need for a transmission system in which distributed amplification may be modified and in particular a system enabling selection of the region of the fiber in which the distributed amplification takes place.